Many important life processes are similar in the bacteria group but not shared in virus group.The drug that block one of these biochemical mechanism in one member of the group will be effective against many other members of the group.But same drug will not work against a microbe belonging to a different group.

Antibiotic commonly block biochemical pathway important for bacteria.they work against many species of bacteria,rather than simply working against one.

For Ex:Many bacteria make a cell wall to protect themselves.The antibiotic penicillin blocks bacterial processes that build cell wall.As a result,the growing bacteria become unable to make cell wall and die easily.

Viruses do not use these pathways at all,that is why antibiotics do not work against viral infections.

If we have a common cold,taking of antibiotic does not reduce the severity or duration of disease.Instead our body secrete an anti viral protein called interferon to combat virus of cold.